[FAB-14536] Clarify private data concept doc
[FAB-14536] Clarify private data concept doc

Add clarifications to the conceptual doc

Change-Id: I18c93fe4d1e454451533676c93320c37e7a811c1
Signed-off-by: David Enyeart <3d699cfec69cc11a9ba1c3a1aef3260ab163489b@us.ibm.com>

xixuejia/fabric,xixuejia/fabric,xixuejia/fabric